Exhaust system are conventionally constructed with passive components, with the exhaust gas flowing through all of them in all operating situations, and with the components together forming the exhaust system. Aside from exhaust gas lines, these components may be a turbocharger, a catalytic converter or a muffler, for example.
In recent times, systems to actively manipulate exhaust noise caused by operating an internal combustion engine and propagating through an exhaust system have been added to such exhaust systems. Such systems impart a characteristic noise to the exhaust noise generated by the internal combustion engine and propagating through the exhaust system. The imparted characteristic noise shall fit the image of a respective manufacturer and be popular with customers. Sound waves produced artificially inside the exhaust system are for this purpose superposed onto sound waves originating in the operation of the internal combustion engine and propagating through the exhaust system (exhaust noises).
This is achieved by providing a sound generator that is in fluid communication with the exhaust system and introduces sound into the interior of the exhaust system. The artificially produced sound is mixed with the sound produced by the internal combustion engine before they exit together through the tailpipe of the exhaust system. Systems of this kind may also be used for sound attenuation. To achieve a complete destructive interference between the exhaust noise sound waves propagating through the exhaust system and the sound produced by the sound generator, the sound waves originating from the loud speaker have to match the sound waves propagating through the exhaust system in magnitude and frequency and show a relative phase shift of 180 degrees. If the anti-noise sound waves generated at the loudspeaker match the sound waves propagating through the exhaust system in frequency and have a phase shift of 180 degrees relative thereto, but do not match the sound waves in amplitude, only an attenuation of the sound waves of the exhaust noise propagating through the exhaust system results.
A respective exhaust system according to the state of the art is described below referencing FIGS. 1A and 1B:
An exhaust system featuring a sound system 1 comprises a sound generator 2 in the form of a soundproofed housing containing a loudspeaker 3 and being connected to an exhaust system 6 in the region of a tailpipe 4 via a sound line 13. Referencing FIG. 1A, a bottom of the sound generator 2 and a bottom of the sound line 13 are disposed above a bottom of the tailpipe 4. The tailpipe 4 has a discharge opening 5 for discharging exhaust gas passing through the exhaust system 6. An error microphone 7 in the form of a pressure sensor is provided at the tailpipe 4. The error microphone 7 measures the pressure variations and thus the noise inside the tailpipe 4 in a section downstream of a region where the sound line 13 enters the exhaust system 6. The region where the sound line 13 enters the exhaust system 6 provides a fluid connection between the exhaust system 6 and the sound generator 2. The term “downstream” hereby relates to the direction of the exhaust gas flow. The direction of the exhaust gas flow is indicated by arrows in FIG. 1B. Between the internal combustion engine 10 and the region providing the fluid connection between the exhaust system 6 and the sound generator 2, a catalytic converter 11 and a muffler 12 are also provided. The loudspeaker 3 and the error microphones 7 are electrically connected to a controller 8. Further, the controller 8 is connected to an engine control unit 9 of an internal combustion engine 10 via a CAN data bus. The controller 8 calculates a control signal for the loudspeaker 3 based on the sound measured with the error microphone 7 and based on the operating parameters of the internal combustion engine 10 received via the CAN data bus in a way that the control signal is adapted to cause a desired overall sound when superposing with the sound propagating inside the exhaust system 6, and outputs the control signal to the loudspeaker 3. The controller may hereto use for instance a Filtered-x Least Means Squares (FxLMS) algorithm, and may try to turn an error signal measured with the error microphone down to zero (in the case of sound cancellation), or to a preset threshold (in the case of sound manipulation) by outputting sound using the loudspeaker.
There is a disadvantage with conventional exhaust systems that corrosive condensate may form and accumulate in the sound generator, sound line or other components of the exhaust system like the catalytic converter or the muffler. The sound generator as well as the sound line are particularly prone to a formation of condensate, since they are usually colder than those components of the exhaust system passed through by hot exhaust gas. Furthermore, the flow rate of the exhaust gas in the sound line and the sound generator is almost zero.
With an accumulation of corrosive condensate in the sound generator or another component of the exhaust system there is a risk of destroying at least one of the sound generator and the sound line and the other component from inside to outside.
To solve this problem, it is known to arrange critical components like sound generator and sound line at a raised level with respect to the exhaust system so that the condensate is enabled to drain. This is also illustrated in the example of FIG. 1A. The possibilities of arranging the sound generator with respect to the exhaust system are hereby considerably limited.
In addition it is known to provide openings on the bottom face of components of the exhaust system, allowing the condensate to drain. Respective openings are, however, only permitted to a very limited degree, since in addition to the condensate also non-treated exhaust gas may escape. Furthermore, the edges of such openings are themselves particularly susceptible to corrosion.